On Wed, 7 Aug 2024, Richard Biener wrote:
> > > This is probably to work around bugs in older compiler versions? If > > > not I agree. > > > > This is deliberate hand-tuning to avoid a subtle issue: pshufb is not > > macro-fused on Intel, so with propagation it is two uops early in the > > CPU front-end. > > > > The "propagation" actually falls out of IRA/LRA decisions, and stopped > > happening in gcc-14. I'm not sure if there were relevant RA changes. > > In any case, this can potentially flip-flop in the future again. > > > > Considering the trunk gets this right, I think the next move is to > > add a testcase for this, not a PR, correct? > > Well, merging the memory operand into the pshufb would be wrong - embedded > memory ops are always considered aligned, no? In SSE yes, in AVX no. For search_line_ssse3 the asms help if it is compiled with e.g. -march=sandybridge (i.e. for a CPU that has AVX but lacks AVX2): then VEX-encoded SSE instructions accept misaligned memory, and we want to prevent that here. Alexander
